In the past, routers have often taken advantage of the rich set of quality of service (QoS) features offered by asynchronous transfer mode (ATM) to implement differentiated classes of service. As Internet Protocol-based QoS capabilities have matured, ATM QoS and Internet Protocol (IP) QoS are now often in conflict.
Internet Protocol QoS features are typically implemented within the routing engine portion of the router. Such QoS-maintaining schemes are also implemented at the packet level. ATM QoS features are usually implemented in ATM segmentation and reassembly (SAR) units, which are typically provided in the router linecards where the router connects to the ATM network. ATM QoS schemes are typically implemented at the ATM cell level, either during the conversion of packet data into cell data or on the cell data itself.
In the case of advanced IP QoS features, such as new packet prioritization features, traditional ATM QoS features often completely undo the router's traffic scheduling features by rescheduling traffic with ATM rules instead of IP rules.
Router vendors have typically used per-virtual circuit (per-VC) flow control to regulate the rate at which traffic is sent from the router routing engine to the ATM SAR device as part of the process of ensuring that the predefined amount of bandwidth is available for designated VC's. This approach effectively regulates the amount of traffic sent by the router (and thus insures the certain QoS), but does not allow the routing engine to control bandwidth allocation in the ATM network. When the linecard applies its conventional ATM bandwidth control mechanisms to the cell traffic, the resulting reordering of cells containing the packet data may result in an entirely different bandwidth allocation when considered at the packet level.